By OCAdmin in Research highlights on

Waterfalls are commonly seen in the mountainous regions of Taiwan. They are often famous attractions that attract tourists to stop by. The formation of waterfall results from the river water flowing over the highly steep riverbed or the escarpment. Are there waterfalls in the ocean interior? To answer this, we should think about does the ocean has rivers. Oh, that’s “ocean current.” Is there any mountain in the ocean? Sure! Obviously, the answer should be yes.

The Kuroshio originates from the North Equatorial Current (NEC). It is a vast river in the Northwest Pacific Ocean. There are often seamounts or islands in its flowing path. The Kuroshio off the eastern coast of Taiwan is a typical example. The Kuroshio in this region has a width of about 100-150 km and a flow speed of 1-1.5 m/s, which is 2-3 times faster than general currents. The I-Lan Ridge runs across the Kuroshio’s path in the water off I-Lan County. To understand the flow instability caused by strong currents encountering the steep terrain above the I-Lan Ridge, the Kuroshio research team formed by Profs. Ming-Huei Chang, Sen Jan, and Yiing Jang Yang in our institute and the oceanographers from Kyushu University and Ehime University in Japan have conducted field observations.

Based on the observations, it is found that the Kuroshio forms a scene similar to land waterfalls after flowing over the sill crest, also resembling the hydraulic jump phenomenon when the reservoir floods. Given a simplified two-layer fluid (see attached figure), the water in the lower layer pours down after passing the crest. Simultaneously, the lee waves are generated at the interface of the two layers, resulting in shear instability, convective instability, and strong turbulent mixing. As a result, the three-layer structure is formed, where the intermediate layer results from turbulent mixing. The above processes have a non-negligible impact on the local marine environment. Notably, these processes are also modulated by tides. Understanding these processes is essential for estimating the conversion rate of ocean energy from large to small scales, improving the turbulence parameterization of numerical models, and assessing the temporal and spatial changes and budget balances of marine organisms and chemical species concentrations. Underwater waterfalls may have no sightseeing value (well, it is the privilege of sea-going oceanographers: )), but they shoulder the responsibility of balancing ocean dynamics.

Please refer to a JPO paper for further information:

https://doi.org/10.1175/JPO-D-21-0245.1